1. Field of Invention
The present invention relates to a method for improving the magnetic properties, especially the watt loss, of an Fe-based amorphous alloy thin strip which is used as the core of an electric-power conversion device, such as a power transformer or a high-frequency transformer, etc.
2. Description of Related Art
Amorphous-alloy thin strip produced by rapid-quenching and solidifying the molten-state alloy has various excellent properties attractive for application purposes. Among the amorphous alloys, an Fe-based amorphous alloy has a high magnetic flux density and a low watt-loss and is hence being used as the material for various cores.
The low watt loss of amorphous alloys, especially an Fe-based amorphous alloy, is believed to be due to the lack of anisotropy, the low hysteresis loss due to lack of defects, such as crystal-grain boundaries and the like, the thin sheet thickness, and the low eddy-current loss due to the high resistivity. The eddy-current loss, in a broad sense, calculated by subtracting the direct-current hysteresis loss from the measured value of watt-loss, amounts to scores to hundreds of times the classical eddy-current loss calculated on the presumption of uniform magnetization. This indicates that the proportion of abnormal eddy-current loss is great in the watt loss because the width of magnetic domains is great and hence the magnetization changes non-uniformly in the amorphous alloy.
In addition, the absolute value of the abnormal eddy-current loss and its proportion in the total watt-loss increase with an increase in thickness, according to studies by one of the present inventors. The sheet thickness of an Fe-based amorphous alloy is usually from 20 to 30 .mu.m. In accordance with recent developments, however, the sheet thickness is being increased, for example, to 40 to 80 .mu.m. To enable the magnetic properties of an Fe-based amorphous alloy to be fully utilized in the case of thin sheet, the abnormal eddy current loss should desirably be decreased.
Several methods are known in the field of grain-oriented silicon steel sheets to decrease the abnormal eddy-current loss. One of them is the scratching method, known, for example, from U.S. Pat. No. 3,647,575, wherein the surface of a silicon steel sheet is scored by means of a hard, pointed end of a tool, ball-pen, or the like to subdivide the magnetic domains. The assignees of this application tried to apply the scratching method to an amorphous alloy thin strip, but did not attain significantly improved results.
Another method is to laser-irradiate the grain-oriented silicon steel sheet so as to subdivide the magnetic domains. However, the laser-irradiating method, and also the scratching method, is not effective when the irradiated grain-oriented silicon steel sheet is stress-relief annealed.
Narita et al report in "Proceedings of 4th International Conference on Rapidly Quenched Methods (1982)", pp 1001 to 1004 the effect of linear strain on the watt-loss, the linear strain being introduced into an annealed Fe-based amorphous alloy thin strip by means of scoring the surface of the strip by means of a diamond needle. According to this report, the strain is effective for reducing the watt-loss at a high-frequency region of 5 kHz or more, but is detrimental to the watt-loss at a low-frequency region of 100 Hz or less. The watt-loss at a low-frequency region is important for a power transformer or the like. Presumably, the ineffectiveness of the strain at the low-frequency region is attributable to the fact that the amorphous alloy inherently has a lower eddy current loss than the silicon steel sheets because of the thin sheet thickness and thus the subdivision of magnetic domains is only slightly effective for decreasing the watt loss. Rather, the strain presumably increases the hysteresis loss and hence the total watt-loss.
In order to decrease the watt loss of amorphous materials, it has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 57-97606 to locally crystallize the materials. This publication discloses to form crystallized regions on the amorphous alloy thin strip along its width in the form of lines or rows of spots. The crystallization methods disclosed are irradiating by laser light or electron beam or conducting current through a metal needle or edge, located in the vicinity of or contact with the thin strip, to heat the thin strip. Japanese Unexamined Patent Publication (Kokai) No. 57-97606 discloses an improved watt-loss at a commercial frequency. Narita et al, who also report formation of linear crystallized regions, allege that such formation broadens the frequency region where the watt-loss is decreased to a low-frequency side, as compared with the scratching method. Nevertheless, according to Narita et al, the formation of linear crystallized regions is ineffective for decreasing the watt-loss or even impairs the watt-loss at a frequency of 200 Hz or less.
Japanese Unexamined Patent Publication Nos. 57-161030 and 57-161031 disclose to irradiate an amorphous alloy by laser light so as to decrease the watt-loss methods other than crystallization. The disclosed methods are effective for decreasing the watt-loss but slightly impair the excitation characteristic. The excitation characteristic is generally represented by the intensity of exciting current required for obtaining a predetermined intensity of magnetic flux density, i.e., an effective exciting current (VA), but is more conveniently expressed by the magnetic flux density (B) induced by a predetermined intensity of magnetic field (H). When the intensity of magnetic field (H) is 1Oe, the excitation characteristic is B.sub.1. It appears that the local strain generated by the laser-light irradiation induces vertical anisotropy and thus impairs the excitation characteristic.
In the case of a grain-oriented silicon steel sheet, the principal aim of applying the scratching or laser-irradiating method to the sheet is to improve the watt-loss characteristic. The impairment of the excitation characteristic due to such application is considered inevitable. In the case of an amorphous alloy, so far as the above-mentioned publications and report are concerned, no improvement of the excitation characteristic by the laser-irradiation is disclosed. If it is attempted to restore the impaired excitation characteristic by means of stress-relief annealing, an effect of the laser upon watt-loss disappears.